Solar energy has the potential to supply the main part of energy demand of future generations. Most photovoltaic solar cells currently produced are based on silicon p-n junctions. However, there is an increasing interest in dye-sensitized solar cells (DSSCs) due to the simple materials and equipment required for their manufacturing, their potentially low-cost production and the adequate and stable conversion efficiencies currently achieved in small-area cells (11.3% in a DSSC smaller than 1 cm2). The development of industrial DSSCs as one of the most promising devices for solar energy conversion is advancing rapidly (Grätzel, 2006).
DSSCs consist of a nanocrystalline, mesoporous network of a wide bandgap semiconductor (the best found was TiO2), covered with a monolayer of dye molecules. The semiconductor is deposited onto a transparent conductive oxide (TCO) electrode, through which the cell is illuminated. The TiO2 pores are filled with a redox mediator, which acts as a conductor, connected to a counter electrode. Upon illumination, electrons are injected from the photo-excited dye into the semiconductor and move towards the transparent conductive substrate, while the electrolyte reduces the oxidized dye and transports the positive charges to the counter electrode.
As stated hereinabove, small-area DSSCs have achieved a conversion efficiency of 11.3% by the Ecole Polytechnique Federale de Lausanne (EPFL) group, but the efficiency of DSSC modules larger than 100 cm2 is still less than 7% (Grätzel, 2006). Scaling up the total device area leads to problems related to efficient current collection (Späth et al., 2003; Dai et al., 2005). The relatively high sheet resistance of both fluorine doped tin oxide (FTO) and indium oxide doped by tin (ITO) layers used as current collectors limits the maximal distance from a photoactive point to a current collector to about 1 cm (Kay and Grätzel, 1996). The practical efficiency of a DSSC strictly depends on the series resistance of the cell that lowers the fill factor. This influence becomes more pronounced in cells with larger area. In order to minimize internal resistive losses in a DSSC module with an area of several cm2 or larger, interconnects must be applied in series connections or as current collectors. At present, a design with a current collector grid applied to the conducting glass or plastic is prevailing in DSSC modules with an area of 100 cm2 and larger.
Metals tested for the grid to reduce resistive losses of transparent conductive oxides on glass and plastic included Ag, Au, Cu, Al, Ni, but all of these metals were corroded by the iodine electrolyte (Tulloch et al., 2004). The only elements that were found stable to corrosion are Pt, Ti, W and carbon (Tulloch et al., 2004; U.S. Pat. No. 6,555,741); however, Pt is too expensive and Ti, W and carbon are too resistive (Tulloch et al., 2004). At present, the material of choice is silver (Späth et al., 2003; Grätzel, 2000; Arakawa et al., 2006; Dai et al., 2005). However, Ag undergoes rapid corrosion in the presence of iodine-containing redox electrolyte and has to be protected by using high quality polymer- or glass-based protecting layers without pinholes. The protecting layer increases the height of the grid and the distance between the electrodes of DSSC that leads to decreased fill factor and cell efficiency. The main method for producing the current collecting silver grid is screen printing.
An additional approach is to use highly conducting substrates, namely, metallic foils (Tulloch et al., 2004) or double-layered TCO consisting of an inner layer of ITO and an outer layer of FTO (Goto et al., 2006). However, metallic foils are not transparent, and illumination from a back side of the cell leads to a decreasing photovoltaic efficiency. On the other hand, double-layered TCO is too expensive.
Upscaling the technology from small DSSCs to large-area modules leads to loss of an active area. At present, the largest disclosed active area for DSSCs modules with a size of 100 cm2 is about 68% (Arakawa et al., 2006). The noneffective area increases the cost of the DSSC. Increasing the active area is one of the goals for DSSC commercialization.